The long-term objective of this proposal is to understand the mechanisms by which Wnt signals are transmitted in a controlled manner and how disruption of this mechanism leads to developmental defects and cancers. The Wnt family of secreted signaling molecules plays pivotal roles in the differentiation and patterning of various tissues during animal development. Aberrant activation of Wnt signaling also causes several cancers. Transmission of Wnt signals is mediated by two cell surface receptors, one from the Frizzled family and the other from the low-density lipoprotein receptor-related protein (LRP) family. It is, however, unclear how Wnt binding leads to the activation of the receptors and how the receptors transmit the Wnt signal downstream. This proposal will evaluate the hypothesis that receptor clustering induced by Wnt binding activates the downstream signal transmission. To establish the mechanisms of Wnt receptor activation, the investigations proposed here will employ biochemical and cell biological approaches to examine how receptors interact with each other and with downstream components in the presence or absence of Wnts. Since both the Wnt and Frizzled families consist of a large number of family members, it remains unclear how individual Wnts achieve specific interactions with various Frizzled proteins. Although differing affinities among various ligand-receptor pairs are thought to play deciding roles in establishing the specificities, this assumption has been difficult to test due to the lack of suitable Wnt preparations. To evaluate this hypothesis, this proposal will directly measure the specificities and affinities of Wnt-Frizzled interactions by employing soluble and active Wnt preparations that have become available recently. Since a fundamental knowledge of the biochemical properties of Wnts is essential to the elucidation of the signaling mechanism, this proposal will also determine the structural and biochemical features of Wnts important for their activities through mutagenesis, protein purification and biochemical characterization. A complete understanding of this signaling pathway will provide important insights for the development of highly selective, mechanism-based approaches for the prevention, diagnosis, and treatment of diseases and cancers caused by aberrant Wnt functions.